Ad space is the area of a website or Web page dedicated to online advertisements. This space is critical for organizations and websites driven by advertising revenue. Ad space pricing is generally determined according to ad format, placement and website traffic.
Techopedia explains Ad Space
A website or blog may have one or multiple online ads. Prime ad space "real estate" is a website's header/footer or sidebar area, which can typically accommodate text, graphics, animation and video. Formats range from text-based links to buttons, banners, banner rotators and skyscraper ads. Traditionally, the 125×125 button above the sidebar is the most popular.
Remnant ad space (also known as remainder advertising) refers to unsold ad space, which may be made available at deeply discounted rates, especially through cost per click (CPC) and cost per action (CPA) advertising.